The design of bifocal eyeglasses makes use of the principle of alternating vision. That is, the eye alternates between two adjacent lenses of different focal power. This approach has not been as successfull in contact lenses, because of the tendency of a contact lens to move with the eye. To overcome this problem, many of the bifocal contact lenses being used today, rely on the phenomenon of simultaneous vision, whereby the eye is presented with two clear images at once. This is achieved, as described for example by DeCarle in his U.S. Pat. No. 3,037,425, by using a central lens, smaller in diameter than the pupil of the eye, surrounded by an annular lens of a differing power. This can be successfull only when the relative brightnesses of the two images remain constant. Unfortunately, in current lens designs, as the pupil of the eye normally changes its diameter, the relative image brightnesses, provided by the two concentric lenses, shift with respect to each other.
A more recent bifocal lens configuration, makes use of a zone plate design as described in my U.S. Pat. Nos. 4,210,391, 4,338,005 and 4,340,283. These designs allow the two different focal powers to be achieved by diffractive effects. However, because these lens designs utilize multiple concentric rings of diameters less than that of the pupil of the eye, as the pupil of the eye opens and closes, only a small number of rings are involved and the relative brightnesses of the two images remain the same.
But even though these lenses maintain a constant ratio of relative image brightnesses between the different focal points, independent of entrance pupil size, this constant is not always satisfactory. We see for example, in FIG. 1, a blazed zone plate bifocal designed according to my U.S. Pat. No. 4,340,283 wherein the zone radii r(k) are given by ##EQU1## with .lambda. the design wavelength, and d the design focal point. In this case the two image brightnesses are equal (i.e. a 1:1 ratio), because the odd zone blaze was chosen to vary linearly to a depth equal to one-half wavelength (.lambda./2) so that the odd zone focal power Fo is equal to 1/d, while the even zones have no blaze in order that the even zone power Fe is equal to zero. This is in accordance with my U.S. Patent wherein it is specified that 1/d=Fo-Fe. However, if we consider FIG. 2, a blazed zone plate with a blaze depth of only one-quarter wavelength, a lens which may be easier to manufacture than the lens of FIG. 1, we see that the image brightnesses are unequal limiting the use of this lens as an ophthalmic bifocal.